Short cycle annealing process for off-composition white iron castings



p 1933- 'l. R. VALENTINE 7 1,925,855

SHORT CYCLE mummne PRocEss FOR OFF-COMPOSITION WHITE IRON CASTINGS Filed July 10, 1951 His Attorney. 5

Patented Sept. 5, 1933 SHORT CYCLE OFF- COMPOSITION INGS ALING PROCESS FOR WHHTE HRCN CAST- Irving R. Valentine, Erie, Pa, assignor to General Electric Company, a corporation of New York Application July 10, 1931. Serial No. 5419.945 ioiaims. (or. 148-218) This application is a continuation in part of my copending application, Serial No. 437,823, filed March 21, 1930, entitled Short cycle anneal of malleable iron.

The present invention relates to a short cycle annealing process for producing malleable cast iron and more particularly to an annealing process adapted to be employed either when the white cast iron from which malleable iron is made has become hard during the usual annealing process or when the white cast iron contains less than the normal content of carbon and silicon. My improved process is also adapted for use in connection with malleable castings containing a relatively small amount of copper.

Ordinarily white cast iron from which malleable cast iron is made has a combined carbon and silicon content which varies from about 3.2% to about 3.6% of the total content of the casting.

For example, if the carbon content is 2.50% the silicon content will be about 0.70% to 1.10% or if the carbon content is 2.30% the silicon content may vary from about 0.90% to 1.30% making a total of 3.2% to 3.6%. It sometimes happens that the total silicon and carbon content of white cast iron is not within this range. If the total content of silicon and carbon exceeds about 3.6% the casting may be annealed in accordance with the process disclosed in my 3o copending application Serial No. 293,276, filed July 16, 1928 now Patent No. 1,830,630. However, if the white iron casting is off-composition or, in other words, has a combined silicon and carbon content less than 3.2%, chilling of the casting takes place very readily causing the formation of a perlitic structure which cannot be broken up easily. Even in the absence of chilling this perlitic structure forms readily in elf-composition castings making it difiicult to anneal them and this difiiculty increases as the total content of silicon and carbon decreases from 3.2%.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. The invention itself however will best be understood from reference to the following specification when considered in connection with the accompanying drawing in which the single figure shows dia grammatically an annealing cycle which may be employed in carrying my invention into efiect.

In my Patent No. 1,830,630, I have disclosed a short cycle annealing process for converting normal composition white cast iron into malleable 65 cast iron. My prior process consists broadly in heating white iron castings to a temperature of about 1000 C., holding the castings at that temperature for about 4 hours or long enough to effect solid solution of the cementite, cooling the castings to a lower temperature zone having an upper limit of about 750 C., and a lower limit of about 675 C. and maintaining the castings in this lower temperature zone until they have normal malleable cast iron characteristics. In order to complete the malleabilizing process in as short a. time as possible the temperature of the castings while in the 675-750 C. temperature zone is varied slightly from time to time. For example, the castings are held at about 740 C. for about 4 hours. The temperature is then dropped to 730 0., held at that point for about 4 hours and again dropped to about 720 C. and held at the latter point for about 4 hours, at the end of which time the castings are completely malleabilized, i. e. they have normal malleable cast iron characteristics. Instead of stepping the temperature downwardly in the 675-750 C. zone, it may be stepped upwardly. For example, the castings may be cooled from 1000 C. to 710 '0. held at the latter temperature for 4 hours, then raised to 720 C. held at that temperature for 4 hours, then raised to 730 C. and held at that temperature about 4 hours at the end of which time the iron is completely malleabilized.

If my prior process is applied to white cast iron 5 having a combined carbon and silicon content less than 3.2% or if the cast iron is chilled while reducing the temperature from about 1000 C. 'to the lower temperature zone so that a form of perlite is produced in the castings which cannot be 90. broken up readily at temperatures below 760 C., it may be found that the castings are not completely malleabilized at the end of the annealing process. However, if such castings are cooled at the end of the cycle to a temperature well below 700 C.; for example to about 500 C. or even to room temperature and the cycle substantially repeated, it will be found that the castings are completely malleabilized at the end of the second cycle.

In carryingout my invention it is desirable to employ at least two furnaces preferably of the electric type and of about five to ten tons capacity. one being employed to heat the castings to tem-, peratures above 750 C. and the other to heat the 105 castings to temperatures. in a zone having limits between 675 C. and 750 C. In operation, the white iron castings'are heated in a high temperature furnace to a temperature of about 1020 C.

This temperature is gradually attained by heating the castings in successive stages as indicated on the drawing. For example, the castings are heated from 500 to 900 C. in a period of about 2 hours, maintained at the latter temperature about 4 hours then raised in an hour to a temperature of about 980 C. The castings are held at the latter temperature 2 hours, raised in onehalf hour to 1000 C., held at the latter temperature about 2 hours and then raised in about onehalf hour to a temperature of 1020 C. The castings are maintained at 1020 C. for about three hours and then cooled gradually to the 675-750 C. zone. The cooling period for loads varying from about 5 to 10 tons is usually from about 6 to 10 hours.

If the castings are cooled from 1020 C. to a temperature of 720 C., the latter temperature is increased gradually in about 2 hours to 730 C. The latter temperature is maintained about 2 hours, then lowered to 720 C. and the castings maintained at this temperature for about 2 to 4 hours. If the initial temperature in the 675-750 C. zone is 740 C., instead of 720 C., the temperature is gradually reduced from 740 to 730 C. in

l about 2 hours. The temperature is maintained at 730 C. for 2 hours and then lowered to 720 C. and the castings maintained at the latter temperature for a period of time varying from 2 to 4 hours.

At the end of the heating period at 720 C. the castings are usually floor cooled to about 400 or 500 C. If desired however they may be cooled to room temperature. The heating cycle is then substantially repeated. For example the castings are heatedin a period of about 3 hours to an elevated temperature which may vary from about 760 C. to 1020 C., maintained at that temperature for about 4 hours and then cooled to the 675- 750C. zone. If the initial temperature in the 675- 750 C. or low temperature zone is 720 C. the temperture of the castings is increased gradually in about 2 hours to 730 C. The latter temperature is maintained for about 2 hours, then reduced to 720 C. and maintained at the latter temperature for about 2 to 4 hours. Likewise, if the initial temperature in the low temperature zone is 740 C. the temperature of the castings is reduced gradually in about 2 hours to 730 C. The castings are held at the latter temperature for about 2 hours.

The temperature of the castings is then reduced to 720 C. and maintained at this point for about 2 to 4 hours. The total time required for the heating and cooling periods will be approximately 50 hours.

Instead of reheating the castings in the second portion of the annealing cycle to 860 C.-or a higher temperature, I may prefer, as a matter of economy, to reheat the castings to a temperature well below 860 C. but above 750 C. Furthermore, changes of temperature in the low temperature zone may be affected in various ways other than those hereinbefore indicated. For example, the second portion of the annealing cycle might comprise reheating the cooled castings to a temperature of 760 C. If the castings have been cooled to 500 C. or lower the temperature may be increased from 500 to 760 C. in a period of about 6 hours. The castings may be held at the latter temperature for about 4 hours then cooled to 0 740 C. in about 2 hours, held at the latter temperature for 4 hours, cooled abruptly to 730 C., held at the latter temperature about 4 hours, cooled abruptly to-720 C. and held at that temperature for about 4 hours to completely malleabilize the castings.

During those portions of the annealing cycle in which the temperature of the load is being increased the temperatures indicated are those at the upper or hottest point of the load whereas on cooling to lower temperatures and while in the low temperature zone the temperatures indicated are the temperatures at the center of the load. When the hottest portion of the load has a temperature of about 1020 C., the coldest portion will have a temperature of at least 1000 C.

It is my present opinion that by cooling the load to a temperature below 500 C. at the end of the first half of the cycle the perlitic formation in the casting is more readily broken up than if the load is reheated directly from 720 to about 760-1020" C. as in my prior application Serial No. 437,823.

The reduction in temperature from the high temperature zone to the low temperature zone may be effected in various ways. For example, the castings may be transferred directly from the furnace in which they have been heated to 1020 C. to a furnace having a temperature of about 740 C., the latter furnace being provided with rapid cooling features such as an auxiliary water cooled chamber fitted with lines through which the hot furnace gases are drawn and cooled, as disclosed in my copending application Serial No. 442,921, filed April 9, 1930. The castings when introduced into the 740 C. furnace should however have a temperature of at least 850 C. or higher. 1

Instead of employing a furnace provided with rapid cooling features the castings may be transferred from the furnace in which they have been heated to 1020 C., to a cooling hood and allowed to remain in the hood until they reach a temperature ofabout 860 C. In transferring the castings to the cooling hood the outer portion of the castings may be cooled to a temperature below 860 C. while the inner portion remains at a somewhat higher temperature. When castings have remained in the cooling hood for about an hour they generally attain a uniform temperature of about 860 C. and may then be transferred to a furnace having a temperature of about 740 C.

It is desirable to cool the castings from 860 C. to 740 C. as quickly as possible without quenching. To accomplish this result the castings may be lowered at intervals from the furnace into the air and then returned to the furnace. Ordinarily, the time required to drop a load from the furnace will vary from about ten to sixty seconds, while about one and one-half minutes will be required to return it to the furnace. After the load is lowered it may be returned immediately to the furnace, or, if desired, allowed to stand in the air for a period of time which may very from about two minutes at the start to about ten seconds at the end of the cooling process. The intervals during which the load is lowered from the furnace may be from about fifteen minutes to one hour apart, preferably one-half hour.

At the start of my process the castings may be heated directly to 1020 C. from room temperature if desired. I prefer however to heat the casting to 1020 C. in successive stages as indicated on the drawing to thereby prevent buming the outer portions of the castings. Likewise, in reducing the temperature from 1020 C. to the low temperature zone there is a tendency with a load of several tons to chill the outside portion of the load unless the temperature is reduced gradually as indicated on the drawing.

While my invention is particularly adapted for use in connection with off-composition white iron castings, it may also be used to advantage with white iron castings of normal composition which contain a small amount of copper, for example, lrom about .l of 1% to 10%. Castings containing about 2.40% carbon, 0.90% silicon, 0.25% manganese, 0.17% phosphorus, less than 0.1% sulphur, from about 0.1% to 10% copper and the remainder iron when given an annealing process such as described in the present application have unusually high tensile strength and elongation. For example, castings having. the above composition would have at the end of the annealing cycle a tensile strength of about 75,000 lbs. per square inch and an elongation of about 20%. 4

What I claim as new and desire to secure by Letters Patent of the United States, is:

The method of malleabilizing white iron cast=- ing having a combined carbon and silicon content less than 3.2% which comprises heating the castings at an elevated temperature above 750 C. to effect solid solution of the cementite, gradually cooling the castings to a lower temperature zone having an upper limit of 750 C. and a lower limit of about 075 8., maintaining the castings in said zone several hours, cooling the castings to a temperature well below the lower limit or" said low temperature zone, reheating the castings to a temperature above 750 0., but not materially higher than 1020 C, holding the castings at said temperature for a few hours, cooling the castings to said low temperature zone and main taining the castings in said low temperature zone several hours.

2. The method of malleabilizing white iron castimrs having a combined carbon and silicon content less than 3.2% which comprises gradually heating the castings to a temperature above 750 C. but not materially higher than 1020" 0., maintaining the castings at said temperature for at least 4 hours, gradually cooling the castings to a low temperature zone having an upper limit of 750 C. and a lower limit of about 075 0., maintaining the castings in said zone for several hours, cooling the castings to a temperature well below the lower limit of said low temperature zone but above 200 C., reheating the castings to a temperature above 750 C. but not materially higher than l020 (7., maintaining the castings at said temperature for about 4 hours. cooling the castings gradually to said low temperature zone and maintaining the castings in said low temperature zone several hours.

3. The method of malleabilizing white iron castings having a combined carbon and silicon content less than 3.2% which comprises heating the castings gradually to a temperature in the neighborhood of 1000 (3., maintaining the castings at this temperature for at least l hours, cooling the castings gradually to a low temperature zone having'an upper limit of 750 C. and a lower limit of about 700 6., maintaining the castings in said zone for several hours, cooling the castin s to a temperature well below 700 (2. but above 200 0., reheating the castings to a temperature above 750 (3. but not materially higher than 1020 C., maintaining the castings at said temperature for about 4 hours, gradually cooling the castings to said low temperature zone and maintaining the castings in said zone for several hours.

l. The methodof malleabilizing white iron castings having a combined carbon and silicon content less than 3.2% which comprises heating the castings at a temperature above 750 C. to thereby effect solid solution of the cementite, cooling the castings in about six hours to a lower temperature zone having an upper limit of 750 C. and a lower limit of about 700 (3., maintaining the castings in said Zone several hours, cooling the castings to a temperature in the neighborhood of 400 C. reheating the castings to a temperature above 750 C. but not materially higher than 1020 6., holding the castings at said temperature for a few hours, again cooling the castings to said low temperature zone and maintaining the castings in said low temperature zone several hours.

IRVING R. VALENTINE. 

